In a continuing search to find methods to contain radioactive high level waste which is a mixture of fission products, fuel components, non-radioactive chemicals, and other radioactive materials, several options have been proposed. When cesium is present, waste forms of borosilicate glass, Synroc, ceramics and concrete exhibit difficulties in processing or high leach rate because of the high solubility of cesium (Immobilization of Cesium into Pollucite Structure by Hydrothermal Hot-Pressing, Yanagisawa et al, Journal of Nuclear Science and Technology, 24[1], pp. 51-60, January 1987). The current optimal melting temperature of borosilicate glass is high, 1150.degree. C. which requires substantial heat energy to achieve. Further, waste forms are generally limited in the amount of cesium that can be contained. Yanagisawa et al. further point out that pollucite (CsAlSi.sub.2 O.sub.6) is one of the more stable phases for cesium immobilization.
Pollucite (CsAlSi.sub.2 O.sub.6) is known to be one of the best materials for Cs containment, exhibiting aqueous leach rates as low as 5.7.times.10.sup.-8 gcm-.sup.2 day.sup.-1 as reported by S. Komareni, G. J. McCarthy, and S. A. Gallagher, "Cation Exchange Behavior of Synthetic Cesium Aluminosilicates," Inorg. Nucl. Chem. Letters, Vol. 14, pp. 173-177, 1978. In the paper, Studies of Pollucite, Vance et al., The Scientific Basis for Nuclear Waste Management, Elsevier 1992, substitution of cations other than aluminum is discussed. Specifically mentioned are Fe.sup.3+, Mn Co Ni or Cr. However, no pollucite phase was observed for Cr. Vance et al. further report that rare earths and uranium may not enter the alkali site. Vance et al. make no statement with respect to any other elements.
In the paper, Fundamental Study on the Solidification of Cs.sup.+ and Sr.sup.2+ With Hydrous Ti.sup.IV Oxide Modified With Si and Zr, Inoue et al., Materials Research Society, 1992, the potential use of Ti.sup.IV -Si oxide for the solidification of radioactive Cs.sup.+. More specifically, Inoue et al. used the protonated silicotitanate Ti.sub.0.52 Si.sub.0.48.2.09H.sub.2 O, H.sup.+ to form. They exchanged for Cs.sup.+ at three loadings, heat treated at 900.degree. C. and obtained the compositions set forth in Table A. The compositions are further shown in a phase diagram of cesium titanium silicate in FIG. 1 as region 10. Inoue et al. report that the dissolution rate of Cs.sup.+ to from these materials becomes very slow after 80 hours of contact, but no equilibrium is reached within 40 days. They further conclude that the cesium titanium silicate is considered a promising material for the solidification of cesium. None of the material phases were identified.
TABLE A ______________________________________ Compositions of Inoue et al., (mole %) Loading Cs.sub.2 O TiO.sub.2 SiO.sub.2 ______________________________________ 1:2.5:2.3 17 43 40 1:2.8:2.5 16 44 40 1:3.2:2.9 14 45 41 ______________________________________
Binary systems of cesium with silica or titania are known. Specifically Cs.sub.2 Si.sub.4 O.sub.9, Cs.sub.2 Si.sub.2 O.sub.5, Cs.sub.2 Ti.sub.2 O.sub.5, Cs.sub.2 Ti.sub.4 O.sub.9, Cs.sub.2 Ti.sub.5 O.sub.11, and Cs.sub.2 Ti.sub.6 O.sub.13 are known binary systems. However, these binary systems have no indication of suitability as containment materials. Binary systems are shown in FIG. 1 as points 12.
Although pollucite exhibits good leach resistance with Cs, and cesium titanium silicate has shown potential as a cesium containment material, there is still a need for a silicotitanate that has a higher loading of titania and a stronger affinity for cesium for cost effective containment of cesium.